The present invention relates generally to the fields of physiological measurements. More particularly, this invention relates to a method and apparatus to position sensors for non-invasively measuring and/or monitoring two or more physiological variables using a miniaturized device that fits in the ear canal of a subject, generating data that is acquired, stored, displayed, and controlled using custom software. There is a particular need for this invention for the continuous monitoring of physiological variables in subjects (such as NASA crew members) who need to be unencumbered and ambulatory.
The present Invention discloses a discrete, non-invasive device capable of measuring/monitoring two or more clinically relevant physiological variables, and unobtrusively providing continuous data for these variables. The raw data thus generated is acquired, stored and displayed to yield meaningful values for clinically relevant physiological variables.
Current commercial practice does not adequately address specific issues that the present invention has been designed to resolve: simultaneous core temperature, arterial blood oxygen saturation, and pulse rate with minimal intrusiveness. Core Body Temperature:
This variable is commonly measured using thermometers that are placed sublingually, rectally, and via the axilla (armpit). Sublingually placed thermometers are not very convenient or comfortable, cannot be left in place for prolonged periods or for continuous temperature measurements, and can be bitten by patients. Rectal thermometers, because of their invasive nature, are generally disliked by subjects who are awake and alert axillary temperature measurements are also inconvenient, requiring the subject to posture unnaturally, and are not as accurate as measurements taken from the ear.
In the 1960s, researchers began to explore the ear canal and its structures (particularly the tympanic membrane, or eardrum) as a site for core body temperature readings. The tympanic membrane shares its blood supply with the hypothalamus (the body""s thermostat), making it a uniquely desirable site for core temperature measurement. Methods for continuously measuring core body temperature via the ear have involved placing a thermistor or thermocouple against the subject""s tympanic membrane/ear canal wall (the sensor makes actual contact with these structures). This procedure has been used in assessing the core body temperature of critically burned patients, who are generally unconscious or anaesthetized. However, this method is not very popular for use in subjects who are awake and alert as it can be uncomfortable, inconvenient, and injurious (possibly resulting in eardrum perforation).
Less invasive technology has been developed, exploiting the fact that thermal energy can be detected and quantified using an infrared sensor Researchers have investigated and developed infrared thermometry as a means to measure infrared emission from the tympanic membrane, without making actual contact with the membrane. This method has numerous advantages, since it is extremely accurate, can be done within seconds, (as there is no need for surface-to-surface thermal equilibrium), does not carry the risk of eardrum puncture, and does not require unnatural posturing by the subject. Moreover, subjects can tolerate the placement of an IR thermometer in the ear canal for long periods of time, allowing continuous measurements of core body temperature. Commercial technology exists for each of the elements of an offset tympanic membrane temperature monitor; however, none of them are known to be able to operate continuously at an ambient temperature different from the measurement site temperature by only a few degrees.
Arterial Blood Oxygen Saturation and Pulse Rate
Pulse oximeters are available to operate by both transmission and reflectance modalities. These devices, which non-invasively measure the oxygen saturation of hemoglobin in the blood, are among the most universally utilized monitoring instrumentation in today""s hospital. Advantageously, the transduced signal also contains beat-by-beat pulse rate information. In many subjects, though by no means 100%, the sinus arrhythmia related to breathing is so pronounced that measurements of changes in the beat-to-beat rhythm of the heart can also be used to derive respiration rate.
However, the present state of the art does little to address the challenge of providing continuous, accurate measurements of multiple physiological variables using a single site on the body, and permitting the subject to be ambulatory and unencumbered.
Therefore, there is a real need for a device that fits in the ear, continuously generating accurate values for multiple physiological variables. Such a device would be very useful in a NASA space-exploration setting, during physical rehabilitation, and in other settings that require a subject to be ambulatory and unencumbered while vital physiological variables are measured/monitored.
The present invention is drawn to a system that acquires multiple variables of meaningful, clinically-relevant physiological data from a single location on the human body in an unobtrusive, non-invasive manner. The invention includes the positioning of sensors relative to one another and to nearby anatomic structures in a system needed to acquire core temperature, arterial blood oxygen saturation, and pulse rate simultaneously and continuously from a location in the human ear canal.
The present invention provides a significant improvement to the prior art, in that it devises a means by which two (or more) clinically relevant variables can be measured using one discrete, unobtrusive device. The invention positions an infrared temperature sensor for measuring core body temperature and a pulse oximeter sensor (xe2x80x9cPOSxe2x80x9d) in a geometry capable of measuring/monitoring the oxygen saturation, heart rate, and core temperature of ambulatory, unencumbered subjects. The invention makes use of miniaturized sensors and circuitry, miniature thermopiles and spectrally matched optoelectronic devices.
Applications of the invention are numerous and include the monitoring of NASA crewmembers during EVA and exercise. The single site monitor allows a new measure of accuracy for continuous core temperature measurements during potential heat stress situations. It will also allow early detection of distress which causes changes in the cardiopulmonary variables of pulse rate and blood oxygen saturation. Commercial applications are also numerous and are generally associated with patient monitoring during surgical procedures, diagnostic procedures, alternate care facility activities, etc. A unique niche market requiring all of the features of the innovation is the monitoring of patients with Chronic Obstructive Pulmonary Disease (COPD).
The simultaneous operation of an infrared temperature sensor together with a pulse oximeter in the ear canal is not a trivial problem. U.S. Pat. No. 5,673,692, co-invented by the present inventors and herein incorporated in its entirety by reference, discloses a basic infrared temperature sensor together with a pulse oximeter in the ear canal, but does not disclose a system for practically solving temperature compensation and sensor interference problems associated with using such a device.
The solution requires some sensor selection, sequencing control, time-division multiplexing, and aiming geometry innovations to prevent thermal interference during long-term monitoring. It also requires precise placement to prevent motion artifacts when patients chew food or speak. Whereas, commercially, it is necessary that the sensor housing fit as wide a range of patient ear anatomical configurations as possible, the use of a custom ear mold to optimize performance of the sensor on NASA EVA crewmembers is desirable.
The approach to the present invention is to solve practical design problems in combining infrared temperature measurements of core temperature from the tympanic membrane with pulse oximetry from the ear canal.
It is an object of the invention to provide a system to position sensors with respect to each other and anatomical structures to prevent thermal interference during acquisition of core temperature, arterial blood oxygen saturation, and pulse rate simultaneously and continuously from a location in the human ear.
It is an object of the invention to position a temperature sensor in a manner that is not vasoactive.
It is another object of the invention to monitor multiple physiological variables from a single site with minimal restriction of patient sensory experiences.
It is another object of the invention to monitor multiple physiological variables from a stabilized location with minimal patient motion artifact generation.
It is yet another object of the invention to monitor multiple physiological variables from a protected location with minimal potential for instrument/sensor damage.
It is yet another object of the invention to monitor multiple physiological variables from a single site with minimal patient objections to site location and ease of use by both males and females.
It is a further object of the invention to monitor multiple physiological variables from a single site using a single ear mold to fit a large range of population.
It is another object of the invention to monitor multiple physiological variables from a single site with simple attachment to a subject and rapid stabilization of data acquisition.
It is another object of the invention to monitor multiple physiological variables from a single site with quick or no calibration requirements.
It is yet another object of the invention to monitor multiple physiological variables from a single site that is useful continuously for at least 24 hours without removal.
It is yet another object of the invention to monitor multiple physiological variables from a single site with low-cost components for commercial feasibility.